A quantity that occurs in the calculation of virial coefficients; it is defined by f=exp(–V2/kT) – 1, where V2 is the two-body interaction potential energy, k is the Boltzmann constant, and T is the thermodynamic temperature. It is related to the second virial coefficient B by: where NA is the Avogadro number and V is the volume of the system. This equation simplified to: in the case of closed-shell atoms and octahedral and tetrahedral molecules. When particles are so far apart that the interaction V → 0, then f → 0 also, but when the particles are so close together that the interaction V → ∞, then f → –1. This enables strong repulsive interactions between particles to be analysed in terms of f but not of V. The function is named after the US physicist Joseph Mayer.